chore(web-example): Added a README for the existing example

[nstawski]

Which problem is this PR solving?

• Adding a README to the web tracing example (Add README.md for tracer-web example #410)

Short description of the changes

• Added a README file with the description on how to run the example, and a screenshot of the running page

[codecov-io]

Codecov Report

Merging #562 into master will decrease coverage by 0.1%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #562      +/-   ##
==========================================
- Coverage   90.22%   90.11%   -0.11%     
==========================================
  Files         151      149       -2     
  Lines        7478     7345     -133     
  Branches      663      658       -5     
==========================================
- Hits         6747     6619     -128     
+ Misses        731      726       -5

Impacted Files	Coverage Δ
...ges/opentelemetry-exporter-zipkin/src/transform.ts	100% <0%> (ø)	⬆️
.../opentelemetry-exporter-zipkin/test/zipkin.test.ts	100% <0%> (ø)	⬆️
.../opentelemetry-plugin-dns/test/utils/assertSpan.ts	100% <0%> (ø)	⬆️
...try-plugin-dns/test/functionals/dns-enable.test.ts	100% <0%> (ø)	⬆️
packages/opentelemetry-web/src/WebTracer.ts	100% <0%> (ø)	⬆️
...entelemetry-exporter-zipkin/test/transform.test.ts	100% <0%> (ø)	⬆️
packages/opentelemetry-metrics/src/types.ts
...ackages/opentelemetry-exporter-zipkin/src/utils.ts
...res/opentelemetry-plugin-pg/test/assertionUtils.ts	100% <0%> (+3.7%)	⬆️
...kages/opentelemetry-plugin-dns/test/utils/utils.ts	60% <0%> (+26.66%)	⬆️
... and 1 more

[dyladan]

Thanks for this @nstawski

Left a few minor suggestions but overall it looks good

[mayurkale22]

Overall LGTM 👍

[mayurkale22]

I will merge the PR, only build failure(docs) is due to david-dm down and unrelated to PR change.

[dyladan]

@nstawski grats on your first merged PR. welcome to the party